1. Field of the Invention
The present invention relates to a Group III-V compound semiconductor represented by the general formula In.sub.u Ga.sub.v Al.sub.w N (provided that u+v+w=1, 0.ltoreq.u.ltoreq.1, 0.ltoreq.v.ltoreq.1, and 0.ltoreq.w.ltoreq.1), and a light-emitting device using the same.
2. Description of Related Art
As a material of light-emitting devices such as ultraviolet or blue light-emitting diodes, ultraviolet or blue laser diodes, etc., a Group III-V compound semiconductor represented by the general formula In.sub.x Ga.sub.y Al.sub.z N (provided that x+y+z=1, 0&lt;x.ltoreq.1, 0.ltoreq.y&lt;1, and 0 .ltoreq.z&lt;1) has been known. Hereinafter, x, y and z in this general formula are sometimes referred to as an "InN mixed crystal ratio, a "GaN mixed crystal ratio" and an "AlN mixed crystal ratio", respectively. Particularly, Group III-V compound semiconductors containing InN in a mixed crystal ratio of not less than 10% are important for display applications because the luminescence wavelength in the visible range can be adjusted according to the InN mixed crystal ratio.
However, the compound semiconductor and light-emitting devices using the same had the following problems.
First, a trial of forming a film of the Group III-V compound semiconductor on various substrates (e.g. sapphire, GaAs, ZnO, etc.) has been made. However, a crystal having sufficiently high quality is still to be obtained because the. lattice constants and chemical properties of the substrates are quite different from those of the compound semiconductor. Therefore, a trial of firstly growing a crystal of GaN whose lattice constants and chemical properties are strikingly similar to those of the compound semiconductor and growing the compound semiconductor thereon to obtain an excellent crystal has been made (Japanese Patent Kokoku No. 55-3834). It has recently been reported that a high-efficacy light-emitting device can be realized by controlling the thickness of a luminous layer to about 20 .ANG. in a light-emitting device comprising a semiconductor represented by In.sub.x Ga.sub.y N (provided that x+y=1, 0&lt;x&lt;1, and 0&lt;y&lt;1) as an active layer (Japanese Journal of Applied Physics, 1995, Vol. 34, page L797). In this case, however, it has also been reported that the luminous efficacy is lowered as the InN mixed crystal ratio of the luminous layer is increased.
Second, the lattice constant of the Group III-V compound semiconductor depends largely on the InN mixed crystal ratio and the lattice constant becomes larger as the InN mixed crystal ratio increases. Therefore, even if a trial of growing the Group III-V compound semiconductor having a large InN mixed crystal ratio on a Group III-V compound semiconductor containing no In (e.g. GaN, etc.) is made, only those having sufficiently small film thickness show good crystallinity. However, it has been known that it is difficult to obtain a crystal having a lattice constant which is largely different from that of a ground layer because of a so-called self regulation effect of the mixed crystal ratio to lattice matching when the film thickness is small. That is, this fact shows that it is difficult to form a thin film of the compound semiconductor having high InN mixed crystal ratio on the semiconductor layer containing no In (e.g. GaN, etc.). Accordingly, it has been difficult to lengthen the wavelength of the light-emitting device by increasing the InN mixed crystal ratio.
On the other hand, as a process of obtaining a light-emitting device having a long luminescence wavelength by using a luminous layer having a low InN mixed crystal ratio, there has been suggested a process of substantially lengthening the luminescence wavelength by applying a tensile stress to a luminous layer in a light-emitting device comprising a quantum well structure using the Group III-V compound semiconductor as the luminous layer (EP-A-0716457 specification). In order to apply the tensile stress to the compound semiconductor having a lattice constant larger than that of the ground layer, however, it can not be avoided to form a lot of misfit dislocations on the interface between the ground layer and luminous layer and, therefore, deterioration of the crystallinity of the luminous layer could not be avoided. The term "misfit dislocation" used herein means a dislocation formed on the interface between two layers because of a difference in lattice constant between two layers laminated to each other.